1. Field of the Invention
The present invention relates to a plug cap for connecting to a spark plug of an internal combustion engine, and more particularly relates to a plug cap configuration which induces less wear of a threaded terminal on the spark plug, and has elements which are resistant to wear.
2. Background Art
Utility Model Laid-Open Publication No. Sho. 63-60288 "Plug Cap" and Utility Model Laid-Open Publication No. Sho. 63-87277 "Attaching Structure for Plug Cap with Integrated Ignition Coil of an Internal Combustion Engine" show conventional plug cap configurations. In FIG. 5 of publication No. 63-60288, a cylindrical member 15 is fixed to a terminal 4a by a pin member 17 meshing with the terminal 4a. A threaded terminal is shown in FIG. 4. In FIG. 3 of publication No. 63-87277 a plug cap is shown which has an integrated ignition coil IC built into a plug cap C. The plug cap C is therefore heavy and the load is borne by a shroud 4 via a seal bar S.
FIGS. 15(a) to (c) are views describing the operation of a conventional pin member. FIG. 15(a) shows a configuration having a straight section 103 of a spring pin housed in a groove 102 of a cylindrical member 101. Member 101 meshes with a screw thread 105 on the terminal side. FIG. 15(b) is a view showing the operation when beginning extraction of the cylindrical member 101. When the cylindrical member 101 is moved upwards, a force in the direction of arrow A acts on the straight section 103. This force is orthogonal to an inclined surface of the screw thread 105, and when the force changes direction to that of direction of arrow B, a horizontal component of this force is generated in the direction of arrow C. The straight section 103 then pushes out towards the left due to the horizontal component of the force in the direction of arrow C. As a result, as shown in FIG. 15(c), the straight section 103 moves as far as the top of the screw thread 105, and the cylindrical member 101 is withdrawn in the direction of the vertically extending arrow.
FIGS. 16(a) to 16(c) are views showing difficulties arising in the use of conventional plug caps. FIG. 16(a) shows depressions 106 that are generated by the hard straight section 103 wearing upon the relatively soft screw thread 105 during long periods of use. As shown in FIG. 16(b), when it is intended to withdraw the cylindrical member 101 upwards, the straight section 103 cannot be moved horizontally (in the direction X in the drawings) by applying force to the straight section 103 in the direction of arrow A, due to the depth of the wear-induced depressions 106.
FIG. 16(c) is an enlarged view of FIG. 16(b). In this figure it can be seen that when the center of the straight section 103 reaches, for example, a point P2 which is further inward than point P1, the straight section 103 cannot now be pushed horizontally. Conversely, if the center of the straight section 103 is further left of or outward from point P1, lateral movement is still possible. However, after long periods of use, it is possible that the center of the straight section 103 will reach the point P2 inward from the point P1. Regarding this point, in the case of a plug cap integrally fitted with an ignition coil as in Publication No. Sho. 63-87277, in order to fix the plug cap to the terminal in a reliable manner, it is necessary to make the spring force of the pin member large. When the spring force is large, the wear of the screw threads occurs after a relatively short period of time.
In the above, a description is given of wear on the side of the threaded terminal of the spark plug, but the same also occurs on the side of the cylindrical member of the plug cap.
FIGS. 17(a) and 17(b) are views showing examples of deficiencies in conventional cylindrical members. FIG. 17(a) shows that the width of the groove 102 is substantially the same as the diameter of the straight section 103. This straight section 103 moves up and down so as to knock against an upper sidewall 107 and a lower sidewall 108 during vibration. As a result, as shown in FIG. 17(b), the sides of the relatively soft sidewalls 107 and 108 are deformed and a so-called tadpole shape is formed. The straight section 103 meshes as a result of movement to the right in the drawings and is released as a result of movement to the left. Movement to the left is therefore indispensable if the cylindrical member 101 is to be detached.
In FIG. 17(b), as the straight section 103 is inserted into a concave part 109, it is necessary to apply quite a large force in order to cause movement in the direction of the arrow 3. The operability of the configuration of FIG. 17(a) is therefore low and this configuration is not preferred. As shown by these illustrations, conventional configurations are seen to develop a considerable reduction in operability after extended use.